Design Of The Cruciform Specimen Of Carbon Fiber-reinforced Composites Under Biaxial Loading

Carbon fiber reinforced plastic(CFRP)wound pressure vessels are widely used for storing and transporting various industrial gases and liquids due to their advantages of light weight,high strength,high temperature resistance and corrosion resistance.Due to the thin-walled axisymmetric structure and withstanding internal pressure,the mechanical responses can be simplified as a mechanical problem under biaxial in-plane loads with different load ratios.However,the present study of mechanical properties of composites mainly focuses on uniaxial stress state.The mechanical behavior of composites under biaxial state is yet mature,and there is no relevant test standard,therefore,it is difficult to reveal the influence law of biaxial stress state on stiffness or strength properties and mechanical behavior of composite materials.In this thesis,the design of biaxial test specimens is the basis to study the mechanical properties of composites under biaxial loading,theoretical analysis and numerical simulation are combined to study the design of cross-shaped specimen of CFRP under biaxial tension with a loading ratio of 2:1 based on the background of CFRP winding cylinders,which lays a foundation for further research on the mechanical behaviors of CFRP under biaxial loading.Based on the grid theory and the general laying principle of laminates,the design of CFRP cruciform specimen laminates with a stiffness ratio of 2:1 in both directions was carried out,and the number of layups and layup order in each direction were determined.In order to ensure the initial damage of the sample in the central test area to promise the validity of the experimental data,four kinds of CFRP cruciform specimens with different thickness ratios were designed,and the key parameters of the central test area were analyzed by using finite element method.The variation law of key parameters with thickness ratios and load ratios are obtained,and the thickness ratio range that meets the requirements is determined.In order to prevent the sample from excessive interlayer stress in the thickness-changing region and unreasonable failure modes,the stress concentration factor is calculated based on the modified Hashin criterion.The influence of the covering layer on the interlaminar stress is compared,and the laying method of sample with minimum interlaminar stress is optimized.Through the sensitivity analysis of the necking angle and the transition arc curvature on the stress concentration factor of the sample center test area,the range of key geometry parameters of the sample which can guarantee the initial damage of the sample in the central test area is determined,and a reasonable design of the specimen is carried out.The specimens of CFRP proposed in this dissertation can be used for biaxial tensile tests of CFRP laminates with bi-directional stiffness ratio of 2:1,and the validity and accuracy of the experimental data is obtained.It can accurately characterize their mechanical behavior under biaxial loading condition,and reveal the influence of biaxial stress state on mechanical properties of composite materials.